Efficiency and battery life have become primary concerns of many mobile device manufacturers. Often, a large portion of the power consumed in a mobile device is due to the radio frequency (RF) power amplifiers used to transmit and receive signals from the device. Accordingly, by reducing the power consumption of the RF power amplifiers of a mobile device, the efficiency and battery life of a mobile device can be substantially improved.
One way to reduce the power consumption of an RF power amplifier is to operate the RF power amplifier in a pulsed mode of operation. In a pulsed mode of operation, an RF power amplifier is powered on and driven to a certain output level in order to amplify an RF signal, then shut down and placed in a state of low power consumption. Although effective for reducing the total amount of power consumed by the RF power amplifier, operating an RF power amplifier in a pulsed state of operation results in a non-linear gain function of the RF power amplifier with respect to the power-on time of the device. Specifically, as the RF power amplifier is powered on, the gain response of the RF power amplifier varies as the temperature of the internal components stabilizes. Due to the stringent wireless communications standards used by many mobile devices, the gain of the RF power amplifier generally does not have time to stabilize after it is powered on before amplification of a signal is required by the mobile device. The resulting gain variation of the RF power amplifier produces non-linear distortion that causes an increase in the error vector magnitude of the signal being amplified. An increased error vector magnitude of an amplified signal may negatively impact the performance and reliability of a mobile device in which the RF power amplifier is integrated.
FIG. 1 shows a graph 10 representing the gain of an RF power amplifier operated in a pulsed mode of operation with respect to the power-on time of the device. As shown by a trend line 12, the gain of the RF power amplifier is non-linear with respect to the power-on time of the device. Specifically, the gain of the device increases in a logarithmic fashion during a “warm up” period 14 of the RF power amplifier, as the internal components of the RF power amplifier stabilize. Accordingly, gain variation may be experienced by a signal amplified by the RF power amplifier.
In order to reduce the error vector magnitude of a signal amplified by an RF power amplifier operating in a pulsed mode of operation, a pulse shaping biasing signal may be applied to the RF power amplifier. FIG. 2 shows conventional pulse shaping biasing circuitry 16 for compensating an RF power amplifier operating in a pulsed mode of operation. For context, supplemental biasing circuitry 18 and an RF power amplifier 20 are also shown. The conventional pulse shaping biasing circuitry 16 includes an input node 22, an output node 24, a resistor-capacitor (RC) ramp signal generator 26 coupled between the input node 22 and the output node 24, and a biasing resistor 28 coupled in parallel with the RC ramp signal generator 26 between the input node 22 and the output node 24. The output node 24 of the conventional pulse shaping biasing circuitry 16 is coupled to the RF power amplifier 20 through the supplemental biasing circuitry 18.
In operation, the conventional pulse shaping biasing circuitry 16 receives a control signal CONT at the input node 22. The control signal CONT may be a square wave voltage, as shown in FIG. 3A. The control signal CONT is delivered to the RC ramp signal generator 26 and the biasing resistor 28. The RC ramp signal generator 26 includes a ramp resistor 30 and a ramp capacitor 32. As will be appreciated by those of ordinary skill in the art, as the control signal CONT is passed through the RC ramp signal generator 26, an inverted ramp signal RAMP is generated, as shown in FIG. 3B. As the control signal CONT is passed through the biasing resistor 28, the amplitude of the control signal CONT is adjusted to produce a square wave signal SQUARE, as shown in FIG. 3C. The inverted ramp signal RAMP and the square wave signal SQUARE are then combined to produce a pulse shaped biasing signal PS_BIAS, as shown in FIG. 3D, and delivered to the output node 24. The resulting pulse shaped biasing signal PS_BIAS can be used to compensate the RF power amplifier 20 for gain variations experienced as a result of operating in a pulsed mode of operation.
The pulse shaped biasing signal PS_BIAS is delivered from the conventional pulse shaping biasing circuitry 16 to the supplemental biasing circuitry 18, where the signal is amplified and subsequently delivered to the RF power amplifier 20. The RF power amplifier 20 includes an RF input terminal 34, an RF output terminal 36, and an amplifying transistor device 38. The amplifying transistor device 38 includes a collector contact C coupled to a supply voltage V_SUPP, a base contact B coupled to the supplemental biasing circuitry 18, and an emitter contact E coupled to ground. The RF input terminal 34 is coupled to the base contact B of the amplifying transistor device 38. The RF output terminal 36 is coupled to the collector contact C of the amplifying transistor device 38. The pulse shaped biasing signal PS_BIAS linearizes the gain response of the RF power amplifier 20 while operating in a pulsed mode of operation by delivering a pulse function that is opposite to the gain variation experienced by the RF power amplifier after being powered on. Specifically, the initial increase in amplitude of the pulse shaped biasing signal PS_BIAS compensates for the initially low gain response of the RF power amplifier 20 as it is powered on. As the gain response of the RF power amplifier 20 increases, the amplitude of the pulse shaped biasing signal PS_BIAS decreases in order to maintain the gain of the device at a constant value.
Although effective at linearizing the gain response and thus reducing the error vector magnitude of signals amplified by the RF power amplifier 20, the conventional pulse shaping biasing circuitry 16 requires relatively large component values to accomplish this task. Notably, the ramp resistor 30 of the conventional pulse shaping biasing circuitry 16 generally must be on the order of 2 kΩ and the ramp capacitor 32 generally must be on the order of 100 nF in order to achieve the appropriate inverted ramp signal RAMP while maintaining the square wave signal SQUARE at a level appropriate for biasing the RF power amplifier 20. The large component values required by the conventional pulse shaping biasing circuitry 16 may consume an unnecessary amount of power and occupy a large area in the circuitry in which they are integrated. Further, the required components are practically incapable of integration due to their size, thereby leading to inefficiencies in the connection and layout of the conventional pulse shaping biasing circuitry 16.
Accordingly, there is a need for biasing circuitry that is capable of stabilizing the gain response of an RF power amplifier operated in a pulsed mode of operation while offering improved efficiency for a mobile terminal in which it is incorporated.